Technical Field
The present invention relates to an offshore energy storage device, storing energy as rotational kinetic energy, comprising an elongate, upright central part, which is rotatable around its longitudinal axis, floating means arranged to keep it upright and floating, a fly wheel arranged concentrically around the central part and anchorage means below the fly wheel for anchoring to the sea bed.
Background Art
Motions in air and water are a concentrated form of renewable energy resources that hold great opportunities to provide energy in an environmentally friendly way. And humans have used these resources for thousands of years. The energy has partly been absorbed and converted by vertical axis power plants, meaning that the part of the power plant that absorbs the energy is located on an axis that is vertically directed, and partly by horizontal axis power plants, meaning that the part of the power plant that absorbs the energy is located on an axis that is horizontally directed.
However the usage of power plants to absorb motions in air and water has been limited, especially in oceans with depths over 50 meters. These areas hold large quantities of energy that is bound in motions but it has been very expensive to install power plants on these locations and large foundations or attachments to the ground have often been used.
A previous power plant that has solved some of these problems is the patent application EP1106825A, which comprises an aggregate to absorb motions in the air where a float body is used to install a wind-power station on top of it. U.S. Pat. No. 4,775,340 is also a similar example of this; here is the floating frame, that the wind-power station is placed on, round in the horizontal plane.
But the power stations must also be stable since water and air flow can vary quickly. A good solution to this is described in the patent US2006269396, where a floating wind-power station floats in a vertical position so that the centre of mass of the power plant is located under the centre of the buoyancy. That power plant is trying to get a good stability through a vertical extent while the U.S. Pat. No. 7,397,144B1 instead discloses a horizontal extent, which makes it possible to build a very large and heavy rotor with no bearings. This patent, U.S. Pat. No. 7,397,144B1, describes an aggregate to absorb the motions in the air where both float body and rotor together rotate as a floating body on the water.
The U.S. Pat. No. 7,397,144B1 discloses a vertical axis power plant, but unlike the previously mentioned power plants, it uses a rotating surface based float body with a horizontal extent. To rotate the body which has a large extent horizontally means, however, that friction will increase because of the higher speed on the outer parts of the float body. The horizontal extent also means that strong forces are acting thereon in stormy weather and large waves, since the surface towards the waves are large, which in turn makes high demands on anchorage and the ability to withstand very large forces. The remaining problems for the device disclosed in U.S. Pat. No. 7,397,144B1 and the previous technologies are still stability and, as a result of this, also anchoring.
Another major problem for renewable energy resources is also that they usually do not have a continuous out-effect, for example, it varies due to wind variations. Being able to store energy in a suitable and cost efficient way is still a major problem. The power plants mentioned above may therefore be positioned at locations with a relatively stable climate, and in combination with other energy systems, for example hydropower, so that a consumer of energy can get a reliable, continuous delivery of energy, even if there is, for example, no wind.
Even if we make use of for example the storage capability of hydro power there is a limit to how many power plants mentioned above that can be incorporated in an energy system. This is one of the reasons for starting to explore different solutions, such as the present invention.